1. Field of the Invention
The present invention relates to an apparatus for generating a tracking error signal for use in a data recording and reproducing system using a disc, and more specifically to a tracking error signal generating apparatus of the sampled-format system.
2. Description of Background Information
On a write once type disk designated as the DRAW (Direct Read After Write) disk for example, a time-division servo signal is recorded as illustrated in FIG. 1. Each sector of the write once disk is made up of 43 servo blocks, and each servo block is formed of two bytes of servo bytes and 16 bytes of data bytes disposed in succession thereto. A servo byte is constituted by two wobbled pits and one clock pit, the wobbled pits being disposed on left and right side of the track center, serving as a marker for the tracking servo operation. When the information detecting point of the pickup (a light spot for detecting information) moves on the track center, the decreases in quantity of light at the left and right wobbled pits becomes equal with each other, while the decreases in quantity of light becomes different depending upon the direction and magnitude of the shift amount when the position of the movement is shifted to left or right. Therefore, a tracking error signal can be generated from the difference between the decreased amounts (the difference between levels of RF signals) at two positions and this tracking error signal is held during the period of the next data signal section.
The distance between two adjacent wobbled pits is varied between longer and shorter distances at intervals of 16 tracks. By sensing the change in the distance, it is made possible even in a high-speed count mode to count the number of tracks correctly (this operation being designated as the 16-track counting).
Furthermore, the distance D between the latter of two wobbled pits and a clock bit is set to a particular distance which does not appear in the data signal section. Therefore, the distance D can be detected as a synchronizing signal. Various timing signals are generated on the basis of the synchronizing signal detected in such a manner. The clock is generated correspondingly to a detection signal of the clock pits. The mirror portion between the pits for the distance D is used as a focus area in which a focus error signal is detected and the focus error signal is held during the period of the next data signal section.
When a DRAW disk having a diameter of 5 inches with such servo bytes recorded thereon, for example, is rotated at 1800 rpm, the pulse generated in the RF signal from the clock pits will have a repetition frequency of 41.28 KHz.
Japanese Patent Application No. 61-198531 (Laid-open No. P63-53760) specifically discloses an example of a recording and reproducing system which is arranged to read the address data and information data recorded on the DRAW disc following the servo signal section, to record the data in a data information part and to reproduce the recorded data.
If It Is attempted to construct a tracking error signal generating part of such a recording and reproducing system by digital circuits for the purpose of downsizing of the system, utilizing a recent advancement of the digital IC technology, a circuit construction as illustrate d in FIG. 2 is conceivable.
In FIG. 4, a pickup (not shown) is arranged to follow a track on the DRAW disc in accordance with the operation of a tracking servo system, to read the recorded information and to provide an RF output signal. The RF signal is converted to a sampled data by means of an A/D converter I operating as a sampling means which performs a data sampling in accordance with a sampling pulse. The sampled data is supplied to a demodulator 2, registers 3 through 5, and to a timing signal generating circuit 11.
The timing signal generating circuit I1 is configured to detect the said sync signal by means of a particular pattern detecting circuit incorporated therein, so as to distinguish the arrival of the clock pit. The timing signal generating circuit 11 generates a system clock which is synchronized with the detection of this clock pit, and supplies the clock to various parts of the circuit (the circuit connections of the system clock are not specifically illustrated), and generates a sampling signal SPC, and sampling pulses SP.sub.1 through Sp.sub.n.
The demodulator 2 is made up of, for example, a 4/15 demodulating circuit, and demodulates the above-mentioned sampled data, to generate a demodulation data which in turn is supplied to a data processing circuit (not illustrated).
The registers 1 through 3 respectively take-in the sampled data in response to the supply of respective one of the sampling pulses SP.sub.1 through SP.sub.3. The sampling pulses SP.sub.1 through SP.sub.3 are generated in order as the information reading spot of the pickup travels over the wobbled pits Pa, Pb, and Pc. With this sequence, read levels of the pits Pa through Pc are respectively stored in the registers 3 through 5. As mentioned before, either one of the pits Pa and Pb is recorded every 16 track turns alternately. The data held in the registers S and 4 are supplied to a comparator 6 in which the values of the data are compared with each other, and a comparison signal indicating a result of comparison is outputted. The comparison signal is supplied, as the 16 tracks count data, to a pickup advancement control circuit (not shown) for example. The data held in the registers 3 and 4 are also supplied to a selector 7. The selector transmits one of the data having a larger value to one of two input terminals of a subtracting circuit 8 in accordance with the said comparison signal. The registers and circuits 3 through 7 constitute a tracking data separating means.
The data held in the register 5 is supplied to the other input terminal of the subtracting circuit 8. The subtracting circuit 8, operating as an error calculating means, is operative to obtain a tracking error data by calculating a difference between the supplied data, and supply it to a D/A converter 9. The tracking error data is converted to an analog signal by the D/A converter 9, and supplied to a tracking servo circuit (not shown) as a tracking error signal after passing through a low-pass filter 10 in which a sampling frequency component of 41.28 KHz is eliminated.
The tracking error signal is generated in this way.
However, the above-described tracking error generating system needs improvements with respect to the following points.
At first, in the tracking error signal generating system of the above-mentioned sampled format system, the sampling frequency is 41.28 KHz when the disk is rotated at 1800 rpm. Since the output signal of the D/A converter 9 shows a characteristic of the first-order hold function, the so-called phase rotation .theta. generated by the sampling under such a condition is expressed by the following equation of: .theta.=-.pi..multidot.(f/41.28) radian, in which f represents the bandwidth of the tracking servo frequency. If the bandwidth of the tracking servo frequency is 3 KHz, then the phase rotation becomes equal 13 degrees. In order to compensate for the phase rotation of 13 degrees, it is necessary to provide a phase compensation circuit in the next stage of the tracking error signal generating circuit.
On the other hand tracking actuators driven by the tracking error signal generally have a high-order resonance characteristic. The high-order resonance characteristic appears, for example, in a frequency range of 10 through 15 KHz, it is desirable to set the cut-off frequency of the said low-pass filter below 10 KHz, so as to sufficiently suppress components of the signal in the resonance frequency band. However, if such a measure is taken, it will cause a problem that the phase rotation appears in the tracking error signal due to the phase characteristic of the low-pass filter.
In addition, the tracking error can be always read as far as the pickup follows the recording track, in the case of a video disc which is a ROM type recording medium. However, with the disk of the sampled format system, the tracking error can be read only in the portion of tracking marker. Thus, the sensed tracking error has a discrete nature, and the quantizing noise is inevitably generated.
Now, a case of performing a track jump during the disk of this type is played, will be considered. If a system in which, as shown in FIGS. 8A and 3B, a jump control signal applied to a tracking actuator of the pickup is changed from a "kick" state for driving the actuator to a braking state for suppressing an swinging movement of the actuator, at a zero-crossing point of the tracking error, there can be a chance that the changeover from the kick state to the braking state is not performed at the proper zero-crossing of the tracking error, and the time period required for converging the jump operation is prolonged.
More specifically, if the zero-crossing of the tracking error occurs at a sampling time t.sub.1 as illustrated in FIG 3C, it is sufficient to effect the change-over of the jump control signal at the zero-crossing point of the tracking error signal. However, when the zero-crossing of the tracking error occurs at a time other than the sampling time t.sub.1 as illustrated in FIG. 3D, the timing of changeover will not correspond to the proper zero-crossing time of the tracking error as illustrated in FIG. 8E if the change-over of the jump control signal is performed at the zero-crossing of the tracking error signal. The variation of the timing of the change-over of the jump control operation is .+-.(1/41.28 KHz) seconds which is equal to the sampling interval.